1. Field of the Invention
The present invention relates to a glass-sealed light emitting element, a circuit board with the glass-sealed light emitting element, a method for manufacturing the glass-sealed light emitting element, and a method for mounting the glass-sealed light emitting element.
2. Discussion of Background
A light source using a light emitting diode (hereinbelow, referred to as LED lamp) is a small and highly efficient light source. The LED lamp is highly reliable since there is no danger of blowout for example. Blue light emitting diodes have been recently developed. A light source for a full-color display can be fabricated by combining blue light emitting diodes and conventional green and red light emitting diodes.
On the other hand, there has been disclosed a method for obtaining white light emission by combining blue light emission and a color conversion material (see Patent Document 1 identified below). White LED lamps obtained by this method have been utilized as the backlights of cellular phones.
FIG. 20 is a cross-sectional view of a conventional LED lamp. A light emitting diode chip 101 is connected to electrodes 102 and 103 by bonding wires 104 and is molded by a resin 105. The resin mold functions to protect the light emitting diode and to control the directivity of emission light. When the resin mold is formed in a shell-shaped resin seal structure, the resin mold has a spherical surface on a leading edge thereof. The directivity of emission light is controlled by adjusting the distance between the light emitting diode and the spherical surface as well as the radius of curvature of the spherical surface. Meanwhile, light emitting diodes have been developed so as to have a short wavelength and a higher luminance.
With respect to having a short wavelength, an attempt has been made to allow light emitting diodes to emit light in the ultraviolet region. By combining such light emitting diodes with color conversion materials for red, blue and green, it is possible to obtain ultraviolet light having good color reproducibility. If a single light emitting diode can achieve to have a high luminance, it is sufficient to use a small number of light emitting diode chips in order to obtain a certain amount of light. Such light emitting diode chips can be used as a light source even in a case where external light is strong as in an outdoor place.
However, there has been a problem in terms of the degradation of a sealing resin used in a light emitting diode. Epoxy resins, which have been conventionally used, are more likely to be decomposed by blue light or ultraviolet light. Transparent resins have discolored to brown, with the result that the amount of emission light is decreased. This is because when an epoxy resin having an epoxy ring with oxygen is exposed to the ultraviolet light, the ring is opened to change in a structure having an absorption property in the visible light region. For recent years, silicone resins have been widely used. The silicone resins can improve the degree of discoloration in comparison with epoxy resins. However, even the silicone resins are subjected to a decrease in the amount of emission light caused by discoloration.
In particular, the decrease has caused a serious problem in light emitting diodes having a short wavelength and a high luminance.
Further, the refractive index of conventional sealing resins is 1.4 to 1.6 while the refractive index of the films and a substrate forming a light emitting diode is as high as 2.4 to 2.5. The light that goes out of a light emitting diode is reflected on a resin interface by the difference in the refractive indexes. For this reason, the light out-coupling efficiency is low.
Furthermore, the conventional sealing resins have a low thermal conductivity, resulting in a poor heat resistance and discoloration due to a temperature increase, and consequently luminance degradation. On the other hand, glass materials are so excellent in light resistance that they are hardly to be degraded by ultraviolet light or blue light. It is possible to produce glass having a high refractive index and a high thermal conductivity by selecting a suitable material composition.
In other words, if a light emitting diode can be sealed by a glass material, it is possible to improve the light out-coupling efficiency and to decrease the problems of the degradation and the heat dissipation of emission light. An example of a molding member made of glass for a light emitting diode is disclosed in Patent Document 2 identified below. In this reference, a glass material is recited as a color conversion member for a light emitting diode, and the glass material comprises one for a window. In this case, since emission light from a light emitting diode directly goes into a medium having a low refractive index of 1, such as air or nitrogen, once, the component reflected by an interface occupies a large part of the emission light, significantly reducing the light out-coupling efficiency. In addition, there remains the problem of heat resistance.
On the other hand, there has been known a technique of sealing GaN based LED element by glass (see Patent Document 3 identified below). An example of this technique is shown in FIG. 21 and FIG. 22. The structure of the example is as follows:
“As shown this figure, a light emitting element 1010 is fixed onto a mount lead 1021 as electric power receiving and supplying means. The bonding wire 1023, is extended between the upper electrodes in the light emitting element 1010 and a mount lead 1021. The bonding wire 1024 is extended between the upper electrodes in the light emitting element 1010 and a sublead 1022 as other electric power receiving and supplying means. As shown in FIG. 22, a cylindrical body 1058a made of low melting glass is provided and put on an assembly 1020 of a light emitting element 1010 and lead parts 1021, 1022. This is placed in an oven to soften the cylindrical body 1058a. As a result, the cylindrical body 1058a covers, in a lens form by the surface tension of the material, the assembly 1020.”
However, since the light emitting element 1010 and the bonding wires 1023, 1024 are all covered with low melting glass in Patent Document 3, there is a possibility that a bonding wire breaks. The sealing member 1058 referred to this reference is likely to be formed in a oval shape since the viscosity of low melting glass is abruptly changed by a temperature change in general. From this point of view, it is supposed that it is difficult to sufficiently increase the directivity of emission light from the light emitting element 1010.
Patent Document 4 identified below discloses a light emitting device, which is capable of avoiding the deformation and the shift of bumps for a light emitting element, and the short circuit between the bumps, which are caused by applying, e.g., a pressure to the light emitting element from a sealing member during sealing.
On the other hand, the inventors have proposed a light emitting device, which is sealed by glass containing TeO2 and ZnO as main components (see Patent Document 5 identified below). In this case, the LED has an average linear expansion coefficient of 85×10−7/° C. while the glass has an average linear expansion coefficient of from 75×10−7 to 140×10−7/° C. Accordingly, it is expected that the residual stress caused after glass-sealing is lower than the one in Patent Document 3. From this point of view, in accordance with the glass disclosed in Patent Document 5, it is possible to reduce the danger of breakage caused by a stress without forming a stress-relaxing portion in a LED as disclosed in Patent Document 3.
Patent Document 1: Japanese Patent No. 3366586
Patent Document 2: JP-A-2003-258308
Patent Document 3: WO-A-2004/082036 Pamphlet
Patent Document 4: JP-A-2006-54210
Patent Document 5: JP-A-2005-11933